This invention relates to liquid crystal displays (LCDs) and, more particularly, to interconnection and addressing schemes which reduce the number of external leads needed to operate an LCD.
Only a dozen years after their introduction, liquid crystal displays now comprise a $500 million market, establishing them as the most important multicharacter display technology after CRTs. World-wide LCD production is well over 200 million pieces annually, largely for consumer electronics applications such as digital wristwatches and credit-card calculators which would be impractical with any other type of display. While most LCDs today are used in such battery-powered portable devices, they are capturing new areas of application because of their improving information capacity, unique optical properties, and their low system cost which arises from batch fabrication and a good match to low-voltage integrated circuits.
The overwhelming majority of LCDs use twisted nematic (TN) liquid crystal (LC) materials which are sandwiched between opposing glass substrates. Segmented transparent electrodes are deposited on the substrates. In the absence of an applied field, the TN-LC molecules form a spiral between aligned surfaces. Polarized light follows the spiral and experiences a 90.degree. rotation. However, the application of only a few volts across the electrodes suffices to destroy the spiral in the region of the LC where the opposing electrodes overlap; i.e., the molecules line up like compass needles in a magnetic field so that the polarization of the light transmitted along the molecules is unaffected. The addition of polarizers and a reflector yields a device which can produce visible patterns by selectively modulating ambient white light. Thus, the LCD achieves high contrast over an acceptable field of view even in direct sunlight, with a power dissipation which is lower by several orders of magnitude than comparable displays which generate light themselves.
In a simple watch or instrument LCD, every individual display segment is connected to its own independent driver. No voltage is applied to unselected segments, while selected segments are turned on as hard as the supply voltage (typically 1.5-3 V) will allow. Because an unrestricted choice of drive voltage maximizes display performance, this simple direct-drive scheme is preferred whenever the number of display segments is not too large.
Direct addressing proves to be too expensive, however, for more complex displays. For example, at least two integrated circuits (ICs) would be needed for direct drive of the more than 64 character segments in the eight-digit calculator display. Matrix addressing provides a solution by allowing each drive line to control several segments. Connecting the segments of a calculator display into a matrix with three rows reduces the number of drive lines from 68 to 28, making possible an LCD calculator with only one IC. Today, most calculator displays are organized as matrices with three or four rows. The same matrix addressing technology is increasingly being used for digital displays in telephones, laboratory instruments and boating equipment. The fact that the display "segment" at each matrix intersection can have a complex shape is widely utilized in electronic games, with pictures ranging from Pac-man.TM. to Popeye.TM..
Unfortunately, the substantial economies of the matrix connection are won only with some sacrifice in simplicity and performance. When many elements are interconnected on the same address line, it is not possible to drive one element arbitrarily hard without inadvertently affecting other elements. An attempt to address a single element by energizing a single row and column may result in "sneak" conduction paths through nominally off LC elements, thereby giving rise to crosstalk. However, the sneak paths can be limited by connecting every lead to a low-impedance voltage source, with none left floating.
Typically, rows are pulsed cyclically while the information to be displayed is multiplexed onto the columns by synchronously choosing the polarity of column voltages to add to or subtract from the corresponding row pulses. It follows that, unlike direct addressing, the unselected elements in a matrix see a non-zero voltage with rms value V.sub.off. In order for unselected elements to appear off, the LCD must have an electrooptic threshold higher than V.sub.off.
It turns out also that the rms voltage V.sub.on which is seen by a selected element is not independent of V.sub.off, rather it is proportional to V.sub.off. Thus, raising V.sub.on also increases V.sub.off so a given segment cannot be driven too hard without causing spurious turn-on of other segments connected to the same matrix lines. Successful multiplexed operation, therefore, requires that the LCD be turned on acceptably by voltages not too far above threshold. FIG. 1 shows schematically the kind of electrooptic characteristic needed for multiplexing. The attainable ratio of V.sub.on to V.sub.off decreases as the number of rows in the matrix increases, because the dwell time for driving any given row becomes a smaller fraction of the addressing period. Clearly, a steeper curve is needed for a matrix with more rows.
In addition, a matrix with more rows, hence more picture elements and higher resolution, means higher addressing cost largely because the number of leads that must be driven increases with the number of resolution elements. For M elements interconnected as a matrix at least 2M.sup.1/2 leads are required. Conversely, N leads can address a matrix which contains no more than 1/4N.sup.2 elements. This limit was recently exceeded by an interconnection scheme which permits N leads to control N(N-1) light emitting diodes (LEDs) through an addressing method based on three-state logic. See, K. Gillessen et al, Proceedings of SID, Vol. 22, p. 181 (1981). Both the reverse blocking characteristic and the sharp threshold for forward conduction of the LED are essential to this approach, so its extension to LCDs is not straightforward.